1. Field of the Invention
The invention concerns a rotary compressor, comprising at least one rotor, which is rotatably mounted in a compression chamber by liquid lubricated greased bearings, and means to inject water on the rotor in the compression chamber.
2. Description of the Related Art
Rotary compressors, in particular screw compressors using two screws as rotors, are often used because of their excellent output and reliability. They deliver almost pulseless compressed air and require little maintenance.
Oil-injected compressors are preferred to the more expensive dry rotating compressors, unless oil-free compressed air is required.
Dry rotating compressors use oil as lubricant solely for the bearings and possibly for the gear box. These compressors can only render oil-free compressed air if the compression chamber is carefully sealed to avoid oil leakage from the bearings to the compression space, which requires complicated and expensive seals.
With oil-injected compressors, oil is injected in the compression chamber so that the oil provides the required lubrication of the moving parts and acts as a coolant. A thin oil layer creates a liquid seal between the rotors and between the walls of the compression chamber and the rotors. In such way, the blow-by of compressed air is reduced, the output is increased and the working temperature is reduced, thereby increasing thermodynamic efficiency.
A drawback of these known oil-injected compressors is, however, the presence of oil traces in the compressed air.
Oil contamination can partly be reduced by means of oil separators and filters; however, only by using complex filtering systems can clean air be obtained.
Filters, in particular the complex filtering systems, are expensive, maintenance intensive, energy absorbing, unreliable and cause a pressure drop.
In practice, fully oil-free compressed air is difficult to obtain in an economically acceptable way with oil-injected compressors.
Therefore, alternative lubricant means other than oil have been investigated.
It is known to replace injected oil in the compression chamber by silicones, more particularly polydimethylsiloxanes.
Nevertheless, these substances have an oily character and are not soluble in water. The removal of traces of silicones from the compressed air is of the same difficult nature as with oil-injected compressors. Complex filtering systems are also required so that silicone-free compressed air cannot be obtained in an economically acceptable way.
In certain applications, compressed air including silicones is not allowed at all. The rather elevated surface activity of the polysiloxane molecules reduces the quality and adhesion of coatings of paints and lacquers that are sprayed with compressed air contaminated with even minimal quantities of silicones.
In compressors used for the compression of hydrocarbons such as ethylene, use of a liquid lubricant composed of polyalkylene glycols which can be mixed with oil is already known.
The solubility of hydrocarbons in these glycols is, however, much smaller than in mineral oils. The solution of hydrocarbons in the mineral oils reduces viscosity, resulting in insufficient lubrication and untimely abrasion.
Polyalkylene glycols do not exhibit this drawback and still are considered oily liquids.
Such glycols have been tested as liquid lubricants for air-compressors. Considering their cost and that traces thereof in the compressed air cannot be tolerated for may applications, their use in air-compressors has not been further developed.
In an attempt to produce oil-free air, compressors have been developed in which water is injected in the compression chamber and oil is used only for the lubrication of the bearings and the gear box.
However, such compressors do have to be provided with reliable and complex seals between the compression chamber and the bearings and gear box. This results in an elevated cost. The usual simple seals cannot be used since leaking oil into the compression chamber reduces the quality of the compressed air and water-leakage in the bearings and possibly in the gear box is detrimental to lubrication and produces corrosion, decreasing the life of the bearings and possibly the gears.
Of course, bearings and gears can be made out of non-corroding high performance materials such as special steel alloys or ceramic materials. Such materials solve the problem of corrosion but are not an appropriate solution to obtain a good tribologically functioning system. Moreover, the cost of such a compressor is considerable.
The aim of the invention is to remedy these drawbacks and to provide a water-injected rotary compressor at a comparatively low price which is long-lasting and produces oil-free compressed air or other fluid.